/*
*        All we need to do is get the socket, and then do a checksum.
*/

int udp_rcv(struct sk_buff *skb)
{
          struct sock *sk;
          struct udphdr *uh;
        unsigned short ulen;
        struct rtable *rt = (struct rtable*)skb->dst;
        u32 saddr = skb->nh.iph->saddr;
        u32 daddr = skb->nh.iph->daddr;
        int len = skb->len;

        /*
         *        数据包至少应有UDP首部长度.
         */
        if (!pskb_may_pull(skb, sizeof(struct udphdr)))
                goto no_header;
                
        /*获取udp首部指针*/
        uh = skb->h.uh;
        
        /* 数据长度，含首部长度 */
        ulen = ntohs(uh->len);
        
        /* 数据包长度不够：UDP长度比skb长度小，意味着数据的丢失，而udp长度比udp首部还要小，好像这个不太可能，除非封包出错 ^o^*/
        if (ulen > len &#124;&#124; ulen < sizeof(*uh))
                goto short_packet;
                
        /* 截去udp报文后面的多余报文 */
        if (pskb_trim(skb, ulen))
                goto short_packet;
        
        /* 开始udp校验和计算，主要查依赖于skb的ip_summumed字段的设置来决定是否需要进行校验和计算 */
        if (udp_checksum_init(skb, uh, ulen, saddr, daddr) < 0)
                goto csum_error;
        
        /* 转换多播或广播处理例程 */
        if(rt->rt_flags & (RTCF_BROADCAST&#124;RTCF_MULTICAST))
                return udp_v4_mcast_deliver(skb, uh, saddr, daddr);
        
        /* 查找数据段对应的socket结构的sk */
        sk = udp_v4_lookup(saddr, uh->source, daddr, uh->dest, skb->dev->ifindex);

        if (sk != NULL) {
                /* 找到了，数据包进入UDP的socket的接收队列*/
                int ret = udp_queue_rcv_skb(sk, skb);
                sock_put(sk);

                /* a return value > 0 means to resubmit the input, but
                 * it it wants the return to be -protocol, or 0
                 */
                if (ret > 0)
                        return -ret;
                return 0;
        }

        if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
                goto drop;

        /* 没有对应的socket. 如果校验和错误，则丢弃它 */
        if (udp_checksum_complete(skb))
                goto csum_error;
        
        /* 发送一个目的不可达报文 */
        UDP_INC_STATS_BH(UDP_MIB_NOPORTS);
        icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);

        /*
         * Hmm.  We got an UDP packet to a port to which we
         * don't wanna listen.  Ignore it.
         */
        kfree_skb(skb);
        return(0);

short_packet:
        NETDEBUG(if (net_ratelimit())
                printk(KERN_DEBUG "UDP: short packet: From %u.%u.%u.%u:%u %d/%d to %u.%u.%u.%u:%u\n",
                        NIPQUAD(saddr),
                        ntohs(uh->source),
                        ulen,
                        len,
                        NIPQUAD(daddr),
                        ntohs(uh->dest)));
no_header:
        UDP_INC_STATS_BH(UDP_MIB_INERRORS);
        kfree_skb(skb);
        return(0);

csum_error:
        /*
         * RFC1122: OK.  Discards the bad packet silently (as far as
         * the network is concerned, anyway) as per 4.1.3.4 (MUST).
         */
        NETDEBUG(if (net_ratelimit())
                 printk(KERN_DEBUG "UDP: bad checksum. From %d.%d.%d.%d:%d to %d.%d.%d.%d:%d ulen %d\n",
                        NIPQUAD(saddr),
                        ntohs(uh->source),
                        NIPQUAD(daddr),
                        ntohs(uh->dest),
                        ulen));
drop:
        UDP_INC_STATS_BH(UDP_MIB_INERRORS);
        kfree_skb(skb);
        return(0);
}